The present disclosure relates generally to drilling holes in large production parts such as those constructed and assembled for aircraft or ships.
When assembling large component parts, such as during aircraft assembly or ship building, typically holes are drilled at various locations in the large parts. Such holes may be referred to as coordination holes because they are used to coordinate the indexing of one component relative to another. Corresponding coordination holes in respective parts also receive fasteners for affixing one part to another part. Each time the same part is replicated, coordination holes must be measured and drilled in the same locations, often requiring precise measurements and accuracy of drilling.
A traditional method of achieving the accuracy for drilling coordination holes in large parts involves using large numerically controlled machines on massive foundations in large controlled environments. Massive foundations are required to achieve stiffness and accuracy for drilling. Large numerically controlled machines operate in five axes to properly locate and orient the holes to be formed in the parts. These machines are also expensive, difficult to set up, not easily moved, and hard to modify. In addition, the weight of a machine and the large work space decrease the accuracy of drilling. In effect, the traditional method of drilling coordination holes in large parts is inflexible.
In addition, the traditional method of drilling coordination holes, using a large numerically controlled machine on a massive foundation, is not suited for drilling coordination holes in higher-level assemblies, such as wing boxes and assembled structures, in part, due to the machine's inability to access smaller or internal recesses and to difficulties in fixturing and indexing large assemblies. Many types of large parts must be drilled to produce coordination holes. Configuring and adapting a large numerically controlled machine for many different parts is time consuming and labor intensive, making using a large numerically controlled machine on a massive foundation an impracticable solution for drilling coordination holes in many types of parts.
In many industries, it is difficult to replace existing components with newer ones due to the difficulty in accurately aligning and drilling mounting holes in the newer component. This is especially true in the aerospace industry. In general, aerospace personnel have difficulty in properly aligning and drilling mounting holes in new components such as aircraft skins when replacing existing ones. For large production runs, special production tools may be designed to properly align and form the mounting holes on new components. However, these production tools can be expensive to design and build. Thus, for limited or small production runs, these production tools are cost prohibitive.
Determinate assembly (DA) is a method of aligning parts using mating physical features. Typically, coordinating holes which are placed on each part or structure are used to take advantage of the ability to install temporary fasteners to hold the parts together. Determinate assembly holes are typically installed on a part or structure during production or via a CNC mill onto a part that is properly indexed in a jig so hole installation is accurate. There are three major challenges to drilling and using DA holes, particularly in large, built-up structures such as wings. First, DA holes are accompanied by inherent reference or indexing tolerance. Tolerance build-up, especially in large parts or in assemblies, can cause DA to be ineffective, particularly when used in large assembly indexing. Second, DA hole placement on parts may not be feasible for built-up structures or for parts that cannot be fixtured properly or easily in a CNC mill (i.e., a wing, wing panel, or fuselage section). For large assemblies where parts are placed relative to one another, DA holes cannot be placed ahead of time because the correct location is not yet known. Lastly, large surfaces (such as a fuselage, wing panel, etc.) can flex during installation or the surface definition may vary considerably and deviate from engineering nominal. Any drill jig applied to the surface without consideration of the as-built surface variation, results in holes with the additional error from jig placement on the as-built surface.
In addition, fixed tooling generally cannot adapt to fluctuations in parts. These differences from nominal dimensions drive errors in tooling position, which cause the operations performed by the tool to also have an error. Some existing solutions include X-Y tables and single eccentric positioners. X-Y tables move in two dimensions, and can reach anywhere within a rectangle defined by the limits of each axis. Typically, X-Y tables are not provisioned to have a hole through the center, which makes mounting a drill bushing cumbersome, and reduces the rigidity when the bushing is mounted on one side. Also, the size of the table is rather large since the axes are coupled, which requires one axis to be placed on top of the other, increasing the height of the table. A single eccentric positioner is only able to achieve positions on a circle, with the radius defined by the offset of the eccentric. Also, rotation of the eccentric has proven to be problematic as torque from drilling can cause the eccentric to rotate, changing the position of the bushing.
There is a continuing need for improvements in means and methods for achieving accurate planar positioning of a tool (such as a drill) within a small envelope. In particular, there is a need for flexible tooling that can adapt to the changes in part features.